This invention relates to improved microwave phase shifters using microstrips fabricated by thin film techniques.
Phased antenna arrays offer flexibility to shape and alter radiation patterns electronically, and achieve such effects as rapid steering of one or more beams, steering nulls, and beam shaping to reduce side lobe effects. Originally developed for radar, phase shifters are becoming of increasing interest for wireless applications, where the requirements are very different.
Recent efforts to improve phase shifters for radar applications have been directed at developing voltage-dependent dielectric devices to replace ferrite phase shifters. See for example, A. T. Findikoglu, Q. X. Jia, and D. W. Reagor, xe2x80x9cSuperconductor/Nonlinear Dielectric Bilayers for Tunable and Adaptive Microwave Devicesxe2x80x9d, IEEE Trans. Appl. Superconductivity, 7, 2925 (1997); L. C. Sungupta, E. Ngo, J. Synowczynski, and S. Sungupta, xe2x80x9cOptical and Electrical Studies of Novel Ferroelectric Composites for Use in Phased Array Antennasxe2x80x9d, Proc. Tenth IEEE Intl. Symp. on Applications of Ferroelectrics (1996), p. 845; F. A. Miranda, R. R. Romanofsky, F. W. Van Keuls, C. H. Mueller, R. E. Treece, and T. A. Rivkin, xe2x80x9cThin Film Multilayer Conductor/Ferroelectric Tunable Microwave Components for Communications Applicationsxe2x80x9d, Integrated Ferroelectrics, 17, 231 (1997).
In a fundamental sense these devices operate as follows. In the paraelectric phase of some dielectrics (above the Curie temperature for a ferroelectric material) there is a large change in the dielectric constant under the application of a sizable electric field (a few volts per micron). If the dielectric is incorporated into a delay line, the electric field can produce a change in phase of a wave propagating along the line.
These phase shifting devices typically have one of two physical configurations. The simplest form is a microstrip configuration where a metal stripline is formed over a ferroelectric body, and the ferroelectric body is sandwiched between the stripline and a ground plane electrode. The microstrip typically has a width of 50 xcexcm to 1 mm, and sufficient length to obtain the desired electrical coupling. The other configuration is a coplanar waveguide where both electrodes are located on the same surface and the microwave propagates along a stripline between the electrodes (see Findikoglu, supra). The coplanar configuration offers the advantage of a lower operating voltage because the drive electrodes can be formed close to the microwave strip line using planar processing techniques.
To illustrate the theory of operation of these devices consider first the delay line equation:
v=c/{square root over (xcex5eff+L )}
where v is the phase velocity and xcex5eff is the effective dielectric constant for propagation on the line. xcex5eff depends in a complicated way on the variable dielectric constant xcex5 of the dielectric. Using the phase shift equation:
"PHgr"=xcfx89l/v
where l is the length of the delay line and xcfx89 is the phase circular frequency. we obtain:
xcex94"PHgr"=(xcfx89l/2c/{square root over (xcex5eff+L )})xcex94xcex5eff
The dielectric constant can be written as a function of applied voltage V or applied field E:
xcex94xcex5(V)/xcex5(0)=a1V+a2V2+. . . 
xcex94xcex5(E)/xcex5(0)=b1E+b2E2+. . . 
The terms of second order give rise to third order intermodulation distortion. The first order terms produce second harmonics which can be filtered out. (Higher order terms in the above equation are usually negligible.) Because of the greater non-uniformity of the electric field for coplanar configured devices as compared with microstrip configured devices, intermodulation distortion can be expected to be greater for coplanar waveguide devices. Accordingly, the microstrip configuration is a better choice. However, the drawback to this device choice is that, due to the thickness of the ferroelectric layer in the sandwich configuration, the voltages required to alter the phase of the propagating wave is very large, i.e. xcx9c500-1000V. Thus there a particular need for improvement of phase shifting devices using a microstrip configuration.
We have developed a new microstrip device using thin film technology where the ground plane electrode is placed on the upper surface of the support substrate, and the ferroelectric layer is formed on the ground plane electrode as a thin film. The stripline is formed on the thin ferroelectric layer. In this configuration the separation between electrodes can be reduced significantly, e.g. by a factor of ten, and the drive voltage can be reduced correspondingly.